A hamete is a trick play, a move that yields an unreasonably big advantage to the player if it is not answered correctly.
Trick play example 1
Trick play example 2
Trick play example 3
Trick play example 4 by aLegendWai
See also: A trick move analysis by Alexandre Dinerchtein. http://www.go4go.net/english/article/tricky/
It's hamete if you knowingly play a sub-optimal move, hoping your opponent will answer wrongly. The best hametes are the ones where if your opponent answers correctly, you only take a very small loss, while if your opponent plays the obvious (and wrong) move, he gets ripped to pieces. :) - deft